The present invention relates generally to an endoscopic objective optical system, and an imaging system using the same, and more particularly to a video endoscopic objective optical system and an electronic endoscope.
For endoscopes, especially those used in medical fields, it is desired to have a wide-angle, wide-field arrangement for the purposes of ensuring ready detection of affected sites in the body cavity, treatment of lesions without oversight, etc. When the endoscopes are used through the abdominal cavity, they need have an angle of field of at least about 70°.
To enhance relay efficiency, an endoscopic optical system is desirously of the telecentric type adapted to allow chief rays to be vertically incident on image relay means. A wide-angle telecentric optical system is achieved by use of a retrofocus optical system made up of a front unit of negative power, a stop and a rear unit of positive power, with the position of the stop in alignment with the front focal position of the rear unit. Basically, however, that telecentric optical system has the nature of producing a lot more negative distortion, because negative distortion occurs through negative refracting power before the stop, and more because negative distortion occurs through positive refracting power after the stop.
Increased distortion is inconvenient and un-preferable for an operator, because there is a great deal of alienation between a real object and an image.
As means for cutting down distortion, Patent Publication 1 shows an endoscopic objective lens of the retrofocus type with less distortion, in which the front unit is constructed of a positive lens and one negative meniscus lens, as depicted in the sectional view attached hereto as FIG. 37.
Further, Patent Publication 2 shows a technique of correcting distortion by use of an aspheric lens.
Furthermore, Patent Publication 3 shows a technique of holding back the occurrence of distortion with no use of any aspheric lens.
With the teachings of Patent Publication 1, however, the convex lens placed on the object side is located nearer to the object side than the concave lens or the distance between them is longer than its distance with the stop, and light rays gain height under the action of the concave lens in the front unit, leading inevitably to an increased outer diameter. The endoscope is an insert; in other words, an increased outer diameter of the optical system attached to its distal portion is not preferable, because of rendering the insert portion huge.
On the other hand, Patent Publication 4 sets forth an example of a hard endoscopic objective optical system with field curvature overcorrected in a positive direction. When a relay lens is used as image relay means, it is known to produce positive field curvature at an objective lens located nearer to an object side with respect to the relay lens, thereby canceling out negative field curvature occurring at the relay lens. Patent Publication 4 is typical of such an objective optical system, as depicted in the astigmatism aberration diagram attached hereto as FIG. 38 (Patent Publication 4, FIG. 6). Nowhere in Patent Publication 4, however, is there any optical system disclosed, wherein an image must be formed on a plane as is the case with a video scope adapted to form an image on a CCD or other solid-state imaging device, and a fiber scope using an image guide, etc.
Further, the type using an aspheric lens as set forth in Patent Publication 2 has some considerable effect on removal of distortion; however, a problem with it is that the fabrication of aspheric lenses generally costs much.
Furthermore, the optical system shown in Patent Publication 3 uses a plurality of lenses for the front unit, and this is again not preferable because of a bit more lenses count leading to increased costs.
Such objective optical systems with reduced distortion and field curvature as described above are suitable for surgical operations under endoscopes. An endoscopic surgery involves making a small bore in the body cavity, and inserting forceps, etc. through the bore for surgery and treatment with a video scope, etc. inserted through the body cavity for getting hold of the field, and has the merit of being lesser invasive than an abdominal operation, finding applications primarily for removal of the gallbladder, surgical removal of the lung in the case of spontaneous pneumothorax, etc.
In the endoscopic surgery, the operator conducts operation while watching a video image on a TV monitor rather than watching directly the affected site, and so images with reduced distortion and field curvature are preferable because they apply no burdens on the operator. For endoscopic surgical operations, a hard video scope of good insert capability is commonly used, and an oblique-vision optical system with the direction of field set obliquely to the front with respect to its longitudinal direction is preferable for use with it. In the oblique-vision optical system, too, there is a need for the operator to easily gain the operating field. FIG. 39 is illustrative of the outward appearance of such a scope (hard endoscope). With an manipulation knob K rotated for instance 90° as indicated by an arrow, the field will turn on a monitor from the before-rotation state of FIG. 40(a) to the after-rotation state of FIG. 40(b).
FIG. 41(a) is illustrative of images on a monitor under endoscopic surgery. As a scope is rotated in its entirety, the image is flipped over with the top and bottom not in alignment with the gravity direction, as can be seen from FIG. 41(b); that is, on the monitor, the top and bottom will be no in alignment with a direction in which biopsy forceps manipulated by the operator appear. This will render endoscopic surgical operations needing meticulous manipulations very awkward.
To overcome such problems, Patent Publication 5 shows a technique of allowing a video scope itself to have a rotating function of turning the field direction without changing the top-and-bottom direction of an operator, as can be seen from FIGS. 42(a) to (c). Specifically, this publication discloses a technique of correcting image rotation by rotation of an image pickup plane 9 of an optical system, as shown in FIG. 43 (Patent Publication 5, FIG. 1).
With a structure, as in Patent Publication 5, of a CCD image pickup plane 9 integrally fixed to the end of a shaft 11 for rotation, however, there is a so-called “play” from errors of internal parts upon fabrication and clearances between parts. As the shaft 11 is rotated, therefore, the center of the field on a monitor makes movement just as in an arc orbit, as depicted in FIG. 44, rendering observation much hard.
Patent Publication 1
JP(A)60-80816
Patent Publication 2
U.S. Pat. No. 4,867,546
Patent Publication 3
U.S. Pat. No. 6,618,207
Patent Publication 4
JP (B) 5-85884
Patent Publication 5
U.S. Pat. No. 6,464,631